1. Field of the Invention
The present invention generally relates to a method of and an apparatus for detecting atrial fibrillation by monitoring and analyzing pulse beats.
2. Discussion of the Related Art
The heart is the major muscle that functions as the primary pump for blood flow throughout the body. The heart contains two upper chambers called atria and two lower chambers called ventricles. The right atrium receives oxygen-depleted blood while the left atrium receives blood enriched with oxygen from the lungs. When the atria are full, the outlet valves within the heart open and the atria squeeze blood into the ventricles. The right ventricle then pumps oxygen-depleted blood to the lungs while the left ventricle pumps oxygen-enriched blood to all parts of the body. In this fashion, the heart functions primarily as a double sided pump.
The heart's internal pacemaker, known as the sinus node, signals the start of each heart beat. This signal originates in the right atrium in the sinoatrial node and travels simultaneously to the left atrium and down to the interatrial septum to the atrioventricular node. The cycle of electrical stimulation that normally occurs is referred to as normal sinus rhythm. The contraction of the ventricles will be referred to as the heart beats.
Many rhythm abnormalities may be present. Atrial fibrillation is one rhythm abnormality in which the atria do not contract normally. Instead, there is a continuously varying pattern of electrical activation of the atria resulting in a rapid highly irregular pattern of impulses reaching the atrioventricular node. The atrioventricular node acts as a filter and allows a reduced number of these impulses to reach the ventricles which results in a highly irregular heartbeat pattern. This irregular pattern has been shown in previous studies to be a random pattern (Bootmsma et al: Analysis of R—R Intervals in Patients with Atrial Fibrillation at Rest and During Exercise. Circulation 41: 783, 1970). Whenever the term “irregular” is used in this application it refers to this random pattern of beats found almost exclusively in atrial fibrillation.
Atrial fibrillation is one of the most common arrhythmias requiring medical attention. Atrial fibrillation may be caused by a number of heart conditions, such as angina, myocardial infarction, heart valve abnormalities, and high blood pressure. These conditions may stretch or scar the atria, thereby causing irregularities in the heart system. Atrial fibrillation may also accompany lung problems or thyroid gland disorders and is also associated with significant morbidity and possible mortality. All persons, young and old, female or male, including the visually and/or sight impaired, may experience atrial fibrillation.
Atrial fibrillation may occur intermittently or chronically. The most serious complication of atrial fibrillation is formation of a blood clot in the left atrium which may result in a stroke. Many people who develop atrial fibrillation, however, are unaware of their abnormal rhythm. Some in the medical profession have, therefore, advocated self screening of the pulse to detect for the possible occurrence of atrial fibrillation. The literature, however, is generally limited to disclosing instructions for manually taking one's pulse accompanied with additional descriptive information.
The reason for using the pulse to detect atrial fibrillation is that the pulse usually corresponds to the heartbeat. The contraction of the left ventricle ejects blood from the heart into the aorta and the resulting pressure wave is detected as a pulse in the arterial system. However, when atrial fibrillation is present, the amount of time between beats varies irregularly.
With a longer time interval between beats, there is more time to fill the ventricles with blood and more blood is ejected by the ventricle in the beat following this long interval. This larger volume of blood in the aorta results in a higher systolic pressure for that beat.
Conversely, when the time interval between beats is short, there is less time for ventricular filling and the volume of blood ejected in the beat following the short time interval is less. This results in a lower systolic pressure for that beat. In some cases, the time interval between beats is so short that the systolic pressure of the following beat is so low that it cannot be palpated as a pulse. A ventricular contraction that cannot be palpated as a pulse in the arterial system results in what is called a “pulse deficit.” This is very common in atrial fibrillation. This pulse deficit means that an irregular pattern of heartbeats in the ventricle may result in a less irregular pulse beat pattern since the shortest intervals between heartbeats may not be detected in the pulse. Therefore, any method used to determine the presence of atrial fibrillation by analyzing the time intervals between beats in the ventricles may not be valid when applied to the pulse beats.
An article by Bert K. Bootsma, Adriann J. Hoelen, Jan Strackee and Frits L. Meijler, entitled Analysis of R—R Intervals in Patients with Atrial Fibrillation at Rest and During Exercise, Circulation, Volume XLI, May 1970 describes an analysis of the time intervals between ventricular contractions using the electrocardiogram. The article evaluates the standard deviation divided by the mean of the time intervals between ventricular beats in normal subjects and in those with atrial fibrillation. The article finds that atrial fibrillation can be accurately differentiated from normal sinus rhythm using this formula. However, this was based on ventricular contractions determined by the electrocardiogram and was not applied to the pulse beat intervals.
Due to the presence of a pulse deficit in atrial fibrillation, results based on ventricular contractions determined by the electrocardiogram may not apply to time intervals determined from analyzing the pulse. Furthermore, the extent of the pulse deficit depends on the method used to determine the pulse beats. A method which detects only pulse beats with high systolic pressures will detect fewer pulse beats compared to a more sensitive method. The more sensitive techniques may be better for detecting more pulse beats but they may also give more false positive readings.
For example, with a photoplethysmograph using a finger probe with a light source and a photoelectric detector, when the sensitivity of the device is increased, the slightest finger movement is detected as a pulse beat. This device at the highest sensitivity setting detects an irregular pulse in those with normal sinus rhythm due to random noise from finger movement. At the highest sensitivity setting, this device would not be useful to detect atrial fibrillation in the home setting. At the lowest sensitivity setting, very few pulse beats would even be detected. Therefore, any device and algorithm which uses the pulse to detect atrial fibrillation must be designed specifically for the purpose of detecting atrial fibrillation.
U.S. Pat. No. 6,095,984 describes an apparatus that can detect “arrhythmia” using the pulse wave. There is no mention of atrial fibrillation in their patent. In fact, the embodiments of their patent that they describe would detect any premature beat, which is a very common rhythm abnormality. Even the most common rhythm disturbance, sinus arrhythmia, which is considered to be a variant of the normal sinus rhythm, would be detected as an arrhythmia by an apparatus constructed in accordance with U.S. Pat. No. 6,095,984. Indeed, U.S. Pat. No. 6,095,984 describes an embodiment (section 1-2-1) where a pulse variation over 0.5% would be detected as abnormal. In sinus arrhythmia, by definition (Braunwald, E. Heart Disease A textbook of Cardiovascular Medicine 1992, p 674) the heart rate varies by more than 10%. U.S. Pat. No. 6,095,984 did not describe methods and apparatus to look for atrial fibrillation and would not be useful for home monitoring of atrial fibrillation, because the algorithm can result in multiple sources of false readings and the apparatus was not specifically set for optimal detection of pulse beats in atrial fibrillation.
An algorithm designed to detect atrial fibrillation by the irregularity of the pulse beat intervals should be designed to reduce the effects of premature beats. One method of reducing the effect of premature beats is to limit the number of beats used in determining the irregularity of the pulse intervals. For example, if premature beats occur on average every twenty beats, then limiting the analysis to only ten beats would reduce the likelihood of a premature beat occurring during the period being analyzed.
There are several devices available that measure both blood pressure and pulse rate, but none of these devices is capable of monitoring, detecting and/or communicating whether or not an irregular heartbeat pattern is present to indicate possible atrial fibrillation. The commercially available devices measure the number of pulse beats over a preset time interval, usually ten (10) seconds, but these devices neither analyze nor determine the presence of irregular heartbeat rhythms.
Commercially available automatic devices for simultaneously determining pulse rates and measuring the blood pressure are available. The most widely used of these devices use the oscillometric or the auscultatory method. In both methods a cuff is placed around an appendage such as an arm and inflated above the systolic blood pressure.
Oscillometry is based on the measurement of the change in the air pressure oscillations in the pressure cuff during cuff deflation. With the auscultatory method, the Karatkoff sounds are recorded when the cuff is deflated. Each of these methods can be used to measure both the blood pressure and to detect the pulse beats.
However, a pulse beat can only be detected by these devices when the systolic pressure of the beat exceeds a minimum value. This minimum value varies with the cuff pressure and the technique used to measure the pressure.
Using the oscillometric method, at higher cuff pressures only heart beats with higher systolic pressures will be detected. For example, a person in normal sinus rhythm with a systolic blood pressure of 120 mm Hg for each beat may have no pulses detected when the cuff pressure exceeds 200 mm Hg. This occurs since the amplitude of the oscillations in the cuff is minimal under these circumstances. However, the device will detect the pulses of another person with a systolic pressure of 180 mm Hg when the cuff is inflated to 200 mm Hg. For the person with a systolic blood pressure of 120 mm Hg, the device begins to detect pulses only when the cuff pressure is reduced well below 200 mm Hg. In this case, it may detect pulses when the cuff pressure is 150 mm Hg or below.
In atrial fibrillation, the time intervals between beats varies irregularly and the systolic pressure of each heart beat also varies. Therefore, when the cuff of these automatic sphygmomanometers is inflated to a high pressure it will only detect pulses with higher systolic pressures. As the cuff pressure is reduced, more pulses can then be detected since the pulses with lower systolic pressures as well as the higher systolic pressure pulses can then be detected. This unique variation in sensitivity to pulse beats means that an algorithm must be designed specifically to detect atrial fibrillation using the automatic blood pressure cuff.
In addition, someone with a normal rhythm may still have a change in his systolic blood pressure from one beat to the next due to the effects of normal respiration. Even though this variation in systolic pressures is usually less than 10 mm Hg, it may be enough to cause some normal sinus beats not to be detected when the blood pressure cuff is inflated to high pressures. Therefore, the algorithm for determining if the rhythm is regular must allow for potential irregular intervals that occur when the blood pressure cuff is inflated to a high pressure.
What is needed is a home monitoring method and apparatus to detect the possible presence of atrial fibrillation and communicate this condition to the user so that the user is alerted to consult a medical practitioner for further testing and/or treatment.
What is also needed is a method that can differentiate an irregular pulse rate pattern from a normal pulse rate pattern and from common heart rhythm patterns that are not of significant risk, such as regular sinus rhythm, sinus arrhythmia, atrial premature beats and ventricular premature beats.
What further is needed is a method of and an apparatus for detecting irregular pulse rhythms during a time period and storing this information so that comparisons may be made with the pulse rate rhythms at later times.
What is further needed is a noninvasive and relatively simple method and apparatus that monitors pulse rate irregularities to detect atrial fibrillation, and that is suitable for use of all ages, and by the hearing and/or visually impaired and that is relatively easy to use.
What is still further needed is a monitoring method and apparatus that detects the presence of irregular pulse beats and then displays and stores: i) the number of irregular pulse beats during a pre-selected time interval; and ii) the duration of time between beats during selected intervals.
Yet another need is for a monitoring method and apparatus that determine whether or not a pulse beat pattern is irregular based on algorithmic or heuristic operations performed on selected pulse beat data.
What is needed is a method and apparatus for detecting the presence of atrial fibrillation by detecting an irregular pattern of pulses using a sphygmomanometer.
What is needed is a method and apparatus for detecting the presence of atrial fibrillation by detecting an irregular pattern of pulses using a plethysmograph such as finger probe with a light source and photodetector.